This invention relates to lithographic printing plates. More particularly, it relates to an aqueous composition-sensitive, photoconductive insulating composition useful in the preparation of electrophotographic elements, to such elements, to a process for preparing lithographic printing plates from such electrophotographic elements and to the lithographic printing plates prepared by such a process.
Electrophotographic reproduction is based on the ability of some normally insulating materials, whose surfaces have been electrically charged, to selectively conduct electric charge upon imagewise irradiation whereby an electrostatic latent image is formed upon such a material. I.e., the charge is conducted away from the surface and through those portions of the insulating materials which have been exposed to the radiation while the areas of the surface which have not been irradiated retain their original charges. The electrostatic latent image is an invisible electrostatic charge pattern generated in a usual exposure procedure, e.g., by lens-projected imaging or contact-printing methods, wherein the charge density at the various areas of the surface is a function of the intensity of irradiation at said areas during exposure. The thus formed latent image may be developed (i.e., made visible) by treatment with a powder (which may or may not be colored and/or mixed with a binding resin) which is attracted to the non-irradiated surface areas which have retained their charges thereby providing a sharp contrast between the irradiated (non-image) and non-irradiated (image) areas. The developing agent is chosen so that after fixation it is resistant to the subsequently applied aqueous composition and prevents attack by said solution on the underlying photoconductive layer. The visible image is then fixed by causing it to become permanently attached to a support on which the image is desired. The support may be the original support to which the composition was applied before exposure or a material to which the image was transferred after development and is chosen in accordance with the desired end use. For instance, if the final product is to be a lithographic printing plate then the supporting material is a lithographically suitable supporting material to which the photoconductive composition is applied and the developed image is fixed directly thereupon. In that case the exposed areas are removed (decoated) from the plate after the image has been fixed thereon, by means of decoating compositions, i.e., aqueous compositions which attack and either dissolve or disperse the exposed photoconductive composition so that it no longer adheres to the support material.
The electrostatic latent image is formed on the surface of an insulating photoconductive layer carried on a conductive support. For example, the free surface of the photoconductive layer is uniformly charged, in the dark, e.g., by application of a corona discharge, and most of the charge is maintained on said surface, due to the insulating character of the layer, in the absence of irradiation. Upon imagewise exposure, however, the conductivity of the layer is greatly increased in the irradiated areas in proportion to the intensity of irradiation. Thus, the surface charge in such areas is permitted to "leak" off while the charge in the irradiated areas is not affected. This pattern of charged and uncharged surface areas is the aforementioned electrostatic latent image.
Electrophotographic materials and processes are of great importance in many areas of the graphic arts industry including the preparation of lithographic printing plates, where they have been found to be preferable to other conventional methods which require additional process steps of preparing a mask or transparency from the original image prior to the exposure step. This requirement has, inter alia, the disadvantage of using a film for the transparency which is expensive (especially if the film uses a silver halide coating) and requiring additional equipment for preparing the transparencies. In the electrographic processes the above step is not required as the image is formed on the printing plate directly from the original resulting in considerable savings in money and time. This increased interest has created a need for suitable materials meeting stringent requirements regarding, e.g., spectral range of use (i.e., range of radiation wavelengths within which material is photoconductive), simplicity of handling, and reliability. In the past such materials have included elemental and inorganic substances such as, selenium, sulfur, zinc oxide, and tellurium and organic substances such as, anthracene and anthraquinone.
The above materials, however, suffer from a number of disadvantages, e.g., in the areas of spectral range of use, ease of handling, light sensitivity and stability.
The above disadvantages have been somewhat obviated by compositions comprising the substituted oxadiazoles described in U.S. Pat. No. 3,189,447 or the substituted oxazoles described in U.S. Pat. No. 3,257,203 which may also comprise water insoluble resins, as well as photosensitizers to increase their sensitivity to the visible areas of the spectrum.
However, said compositions suffer the disadvantages of poor resolution and sensitivity to the "pre-exposure effect" and high material costs.
"Sensitivity to the pre-exposure effect" refers to that property of some insulating photoconductive materials whereby their capacity to be charged, as well as their photoconductivity upon irradiation is temporarily decreased upon exposure to radiation, prior to the electrostatic charging. These decreased capacities return to their normal values after the elements have been stored in the dark for a period of time. The need for such "dark storage" is disadvantageous either with respect to an increase in "down-time" or by requiring an excess of inventory.
Additionally, the elements comprising the above photoconductive compositions can only be decoated using organic solvents. At present, when the environmental impact of all industrial processes and materials is being critically scrutinized, that factor can be an extreme limitation on the use of such compositions and provides an incentive for greater research efforts to produce elements which can be decoated by means of aqueous compositions.
The instant invention overcomes the aforementioned disadvantages by providing a low cost composition which has good spectral sensitivity, good speed, good resolution, is not subject to the pre-exposure effect, and is decoatable, by aqueous compositions.